The present invention relates to thermistors which measure the surface temperature of electronic devices and which are used in temperature compensation for the same. More particularly, the invention relates to chip-type thermistors, such as those adapted for surface mounting on printed circuit boards.
A prior art chip-type thermistor includes a thermistor element having silver-palladium electrodes fused at both ends thereof. The palladium imparts soldering heat resistance to the electrode, thereby preventing the silver from dissolving when soldering a chip-type thermistor to a substrate.
A drawback of the prior art is that palladium decreases the solder adhesion of the electrode to the substrate, thereby establishing an upper limit on the amount of palladium which can be used. When soldering the electrode at high temperature continues for a long period of time, however, limit amount of palladium is insufficient to impart adequate soldering heat resistance to the electrode.
The prior art thermistor improves soldering heat resistance and soldering adhesion by providing a plating layer on the surface of the electrodes, as in the case of a chip-type capacitor. A drawback of this technique is that, since a thermistor element is electrically conductive (unlike the capacitor), plating a conductive material directly on the surface of the thermistor element alters the resistance value of the thermistor element from the desired or expected value. In addition, a portion of the thermistor element is eroded by the plating liquid, thereby reducing the life and reliability of the thermistor.
Referring to FIGS. 10, 11(a) and 11(b), Japanese Laid-Open Patent Publication No. 3-250,603 discloses a chip-type thermistor 5 which attempts to overcome the above drawbacks. A thermistor element 1 includes a glass layer 2 covering all but the ends of thermistor element 1. An electrode layer 4 is baked on the ends of thermistor element 1. Glass layer 2 has a softening point approximately equivalent to the baking temperature of a baked-on electrode layer 4. A protective plating layer (not shown) covers baked-on electrode layer 4. The protective plating layer may be, for example, nickel.
Although chip-type thermistor 5 has good solder adhesiveness, good solder heat resistance and could decrease discrepancies in resistance values, problems occur because the softening point of glass layer 2 is approximately the same as the baking temperature of baked-on electrode layer 4.
Referring now also to FIGS. 11(a) and 11(b), glass layer 2, at the edge of thermistor element 1, softens when baked-on electrode layer 4 is baked on to glass layer 2 and thermistor element 1. This permits glass layer 2 to flow easily downward from the edge. In extreme cases, glass layer 2 disappears from the edge area and causes thermistor element 1 to be left exposed. In addition, the shape of glass layer 2 is often distorted during processing.
Referring specifically to FIG. 10, another problem is that during the baking on of baked-on electrode layer 4, thermistor element 1 may be placed on baking tools such as a baking platform or a baking sheath. Furthermore, a group of chip-type thermistors 5 can be baked at the same time. This can cause glass layer 2 to melt and stick to the baking tools or to other chip-type thermistors, leaving a contact mark or a melt mark 3 on glass layer 2.
Referring to FIG. 11(b), a further problem is that the glass frit, which is melted to form baked-on electrode layer 4 reacts with glass layer 2. The glass frit melts into glass layer 2 and, in extreme cases, both glass layer 2 and baked-on electrode layer 4 flow away at the edge of thermistor element 1, again, leaving thermistor element 1 exposed.
Japanese Laid-open publication No. 3-250604 discloses a thermistor made of a glass containing crystals of inorganic compounds such as alumina, zirconia and magnesia. The glass and the inorganic crystals are mixed together in a powder state. An organic binder and solvent are added to this mixture to create a paste. This paste is printed and baked onto the thermistor element, forming a glass layer. The above-noted problem is solved because the presence of the inorganic crystal powder in the glass layer of this thermistor increases the softening point of the resulting glass layer as compared to the glass layer for the thermistor formed by Japanese Laid-open publication No. 3-250603.
A drawback of the thermistor made by Japanese Laid-open publication No. 3-250604 is that it is difficult to mix uniformly the inorganic crystal powder and the glass powder. The resulting paste is difficult to print on to the thermistor element and results in non-uniform distribution over the surface of the thermistor element.
A further drawback is that bubbles are formed and remain in the glass layer because of the presence of the inorganic crystals. The bubbles tend to burst and become open pores. This allows plating fluid to infiltrate into the pores during the plating process. The plating fluid erodes the thermistor element and decreases the reliability of the thermistor. Finally, the surface of the glass layer becomes irregular and uneven due to the baking on of the baked-on electrode layer. This damages the appearance and changes the expected resistance value of the thermistor.